1. Field of the Invention
The invention relates to electronic semiconductor devices and more precisely to devices comprising at least one planar, P-N junction such as, for example, diodes, bipolar transistors and integrated circuits designed to operate at high voltages, i.e. at voltages of some thousands of volts.
2. Description of the Prior Art
It is known that in order to achieve and maintain high voltages in semiconductor devices with planar, P-N junctions, current technology provides various possibilities. All these have the common objective of providing a junction which comes as close as possible to the ideal case of a junction with plane, parallel surfaces having an infinite extension.
A known device, disclosed in the publication "Solid State Electronics", 1972, Vol. 15, pp. 93-105, involves the use of a metallic field plate which extends over a layer of silicon oxide of constant thickness above the edge of a planar junction. The effect of this structure, when the junction is reverse biased, is to widen the space charge region below the field plate and to increase the radius of curvature of the equipotential lines, thereby causing a reduction of the electrical field and consequently an increase of the breakdown voltage. However, in this case the breakdown voltage is restricted to 600 V as a result of the edge effects which the field plate introduces into the junction.
The above publication also describes a planar junction structure with the metallic field plate disposed on a layer of silicon oxide of variable thickness which enables the above-mentioned voltage limit to be exceeded by modifying the boundary conditions and thereby increasing the breakdown voltage to a value of approximately 1000 V.
A further known possibility is to modify the terminal portion of the junction.
This technique, known by some as "Junction Termination Extension" (JTE) or by others as "Implanted Field Plate" is described, for example, in the publication "IEEE Transactions on Electron Devices", 1983, Vol. ED-30, pp. 954-957. FIGS. 10 and 11 in particular of this publication show a P-N junction terminating with two implanted zones. This structure makes it possible to reach a breakdown voltage of 1400 V, using an N doped substrate with 1.15.times.10.sup.14 atoms/cm.sup.3 in which an impurity of p-type is diffused at a concentration of 10.sup.17 atoms/cm.sup.3 and in which two JTE zones with surface concentrations of 3.times.10.sup.15 and 1.times.10.sup.15 atoms/cm.sup.3 are respectively implanted.
Nowadays, stable, reliable and economically viable electronic semiconductor devices with P-N junctions capable of withstanding voltages of some thousands of volts are required. These devices are required, for example, in the fields of high voltage supply devices, radar, electromedical apparatus using X-rays and, in general, in any field requiring the use of high voltages.